Electrochemical behavior and structural change of spinel-type Li[LixMn2−x]O4 (x=0 and 0.2) in sodium cells

Abstract

Electrochemical behavior of spinel-type Li[Li_xMn_2−x]O₄ samples is examined in sodium containing non-aqueous electrolyte solution. Li ions in stoichiometric LiMn₂O₄ sample are electrochemically extracted in sodium containing electrolyte solution, forming Li_xMn₂O₄ (x = 0.1) by first oxidation process. Sodium ions are inserted into the lithium extracted spinel phase, which results in the phase transition. From structural analysis by X-ray diffraction and phase transition behavior in the sodium cells, it is found that an original spinel phase in part converts into a layered phase, Na_yMnO₂. Thus prepared Na_yMnO₂ can deliver approximately 190–200 mAh g⁻¹ of reversible capacity with relatively good capacity retention as positive electrode materials for rechargeable sodium batteries. Nonstoichiometric Li-rich manganese spinel, Li[Li_0.2Mn_1.8]O₄, also accommodates the sodium ions, similarly to the stoichiometric phase. However, phase transition to the layered phase is not observed. Instead, the sodium insertion into Li[Li_0.2Mn_1.8]O₄ induces strain in the crystal lattice.